Reflected or backscattered light in optical fiber transmission lines caused by the presence of optical components in the lines results in poor transmission performance. The backward propagation of light can damage components at the low-power end of fiber amplifiers. Backward light is amplified both by the gain medium and, for narrow-spectral-band amplifiers, by stimulated Brillouin scattering (SBS). The backward light generated by SBS is down-shifted in frequency by the SBS Stokes frequency but is still within the amplifier gain bandwidth. Optical isolators of various designs are used to eliminate the adverse effects of returned beams. Narrow-band amplifiers are needed for coherent beam combination by active phasing of multiple amplifier chains. A high-power fiber-amplifier system can require multiple isolators, both in series within a chain and in parallel if there are multiple chains. The isolators currently available for use with fiber lasers and amplifiers are limited to about 10 Watts power and have significant loss. These isolators are inadequate for fiber lasers and amplifiers currently reaching several kilowatts of power. Using bulk isolators and re-injecting light into fiber leads to undesirable alignment sensitivity.
An isolator consists of a Faraday rotator sandwiched between two linear polarizers, and may also include birefringent material. Although the Verdet constant V=1.28 (rad/T m) (μm/λ)2 characterizing the strength of Faraday rotation is small in fused silica, it is nevertheless possible to achieve the required 45° Faraday rotation in a silica fiber (J. L. Cruz, M. V. Andres, and M. A. Hernandez, “Faraday effect in standard optical fibers: dispersion of the Verdet constant,” Appl. Opt. 35, 922-927 (1996)). The Faraday rotation is the product of the Verdet constant, the effective longitudinal magnetic field, and the length of the magnet array. Since the magnet array producing the Faraday rotation is sizable, it is highly desirable that a single magnet array be usable for multiple isolators.
Turner and Stolen succeeded in producing a 45° Faraday rotation of the polarization of 632.8-nm and 830-nm light in birefringent fiber (E. H. Turner and R. H. Stolen, “Fiber Faraday circulator or isolator,” Opt. Lett. 6, 322-324 (1981)). For the 632.8-nm work the fiber was looped 9 times through slots in an array of 14 samarium-cobalt magnets with alternating polarity. The period of the array was matched to the 3.3-cm beat length of the polarization modes. This may be termed magnetic quasi-phase matching. About 2 meters of the fiber was in the magnetic field; an additional 5 meters of fiber was in the loops. The magnet configuration was far from optimal, since the magnets were 0.53 cm long and separated by longitudinal gaps of 1.12 cm.
Lafortune and Vallée rotated the polarization of He—Ne laser light by 45° in only 40 cm of standard single-mode fiber (J.-F. Lafortune and R. Vallée, “Short length fiber Faraday rotator,” Opt. Commun. 86, 497-503 (1991)). They used an array of 36 pairs of block-shaped NdFe magnets. The magnets within each pair were separated by a transverse gap and were oriented with the same longitudinal polarity. Alternate pairs were arrayed longitudinally and periodically with opposite polarities. The fiber passed longitudinally through the center of the transverse gap. The magnet pairs were separated by longitudinal gaps in order to insert glass plates about 1.7 mm in width which pushed transversely on the fiber to produce local birefringence. The pressure on the plates was adjusted to produce ½-wave of phase advance between the horizontal and vertical polarizations at each contact point. This produced a cumulative increase of the Faraday rotation in each segment. The output typically achieved a polarization intensity ratio of 25 to 30 dB. However, there were several disadvantageous features to this approach. First, it was necessary to strip the polymer cladding from the fiber in order to maintain constant pressure at the contact points. Second, there were average losses of 0.02-0.03 dB per contact point. Third, this configuration does not permit longitudinal displacement of the fiber when the glass plates are inserted or insertion of additional fibers into the transverse gap, which would allow a single magnet array to provide the Faraday rotation for multiple isolators.
Forty-five degree tilted fiber Bragg gratings written in the core of photosensitive fiber are suitable as polarizers for high-power in-fiber isolators. These gratings are under development at Aston University, Birmingham, UK. They transmit the p-polarization, while scattering the s-polarization out the side of the fiber. Writing a fiber grating requires the removal of the polymer jacket on a short section of the fiber. Zhou et al. reported an in-fiber polarizer with a polarization-extinction ratio (PER) of 33 dB near 1550 nm wavelength with a 5-cm grating (K. Zhou, G. Simpson, X. Chen, L. Zhang, and I. Bennion, “High extinction ratio in-fiber polarizers based on 45° tilted fiber Bragg gratings,” Opt. Lett. 30, 1285-1287 (2005)). More recently, sponsored by US Air Force funding, this group achieved a 45 dB PER near 1099 nm. A theory of tilted fiber gratings indicates that the transmission of the p-polarization through 45° gratings is excellent (Y. Li, M. Froggatt, and T. Erdogan, “Volume current method for analysis of tilted fiber gratings,” J. Lightwave Technol. 19, 1580-1591 (2001)).